The present invention generally relates to the field of photolithography, and more specifically relates to a photo lithographic mask made of a flexible, transparent material and having internally reflective optical interfaces that reflect a portion of an excitation optical signal and regions through which light may propagate for imparting a pattern in a photo resist layer formed on a silicon wafer.
The circuit-on-a-chip industry has been characterized since its inception in the 1960s by the production of chips having ever higher device densities. High densities demand high precision in the laying out of the devices and their interconnections on the semiconductor chip. As the densities have increased, so has the degree of precision demanded. For many years, the dominant response to these demands has been to use photo resist-based lithography. Lithography involves exposing an image of the circuit in a photo resist layer formed on a silicon wafer by shining ultraviolet (UV) light on the photo resist through a masking device having slits that form a desired pattern. Subsequent steps in the fabrication process then depend on those portions of the photo resist that had been illuminated having different physical and chemical properties than those portions that had not been illuminated. As a general proposition it can be noted that manufacturing efficiency is improved by investing the effort required to produce a mask and then using that mask to produce large numbers of chips. The more chips that can be manufactured using a given mask, the more industry can afford to invest in a particular mask. Because of this, techniques are available for forming the patterns on the mask that would be impractical if applied directly to the individual chips.
One type of recently developed mask is made of a flexible material that is placed in direct contact with a photo resist layer formed on a silicon wafer. An important advantage of a xe2x80x9cdirect contactxe2x80x9d mask is that the need for expensive focusing optics is obviated. The mask includes opaque regions defined by an opaque layer typically formed using ink and optically transparent regions which collectively define a mask pattern. When light is directed through the mask, some light is occluded by the opaque regions and other light is allowed to propagate through the mask. Only photo resist exposed to the light is developed. In this way, the mask pattern is replicated in the photo resist layer. One limitation of such a mask is that the occluded light is absorbed as heat energy by the mask material. The amount of energy absorbed by the mask in a given time period must be less than that which would damage the mask material, thereby placing a limit on the intensity of the light directed through the mask, thereby limiting the rate at which microcircuits may be manufactured. Another disadvantage of the occluding layer is its vulnerability to forming pinholes due to mechanical flexure and from optical damage induced from irradiation from intense optical sources such as a laser. A need therefore exists for a mask which is less sensitive to the thermal limits of the mask material, and to pinhole damage.
The present invention provides a photo-lithographic mask which includes a flexible, optically transparent body having an optically transmissive first surface for receiving an optical signal, and a second surface opposite the first surface having grooves for internally reflecting first portions of the optical signal and for allowing second portions of the optical signal to be transmitted through the second surface when the second surface is pressed against a wafer. The body consists essentially of silicone. The grooves have a saw tooth profile that are configured at an angle that exceeds the critical angle of the silicone-to-air interface with respect to the direction of the incoming optical signal.
In a second embodiment of the invention, a photo-lithographic mask includes a flexible, optically transparent body having an optically transmissive first surface for receiving an optical signal, and a second surface opposite the first surface. The second surface has grooves for internally reflecting first portions of the optical signal and contact areas generally parallel to the first surface for allowing second portions of the optical signal to be transmitted through the second surface.
An important advantage of the invention is that optical energy that is not required to expose photo resist, is reflected back out of the element so that the element does not suffer damage from absorbing excessive heat energy. Another advantage of the invention is that it obviates the need for applying an ink or occluding layer as a light blocking mechanism. These and other advantages of the invention will become more apparent upon review of the accompanying drawings and specification, including the claims.